Light olefins, defined herein as ethylene, propylene, and butylene, serve as feeds for the production of numerous chemicals. Olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Alternative feedstocks for the production of light olefins are oxygenates, such as alcohols, particularly methanol, dimethyl ether, and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
The catalysts used to promote the conversion of oxygenates to olefins are molecular sieve catalysts. Because ethylene and propylene are the most sought after products of such a reaction, research has focused on what catalysts are most selective to ethylene and/or propylene, and on methods for increasing the selectivity of the reaction to ethylene and/or propylene.
ZSM-5 was the first and most extensively studied catalyst for the conversion of methanol to olefins. Unfortunately, ZSM-5 produces not only the desired light olefins, but also undesired by-products. In particular, ZSM-5 produces aromatics, particularly at high methanol conversion. Catalysts are needed which do not produce large amounts of unwanted by-products, such as aromatics, methane, carbon monoxide, and hydrogen gas.
Zeolites with a small pore size have a higher selectivity to lower alkenes, even at 100 mol % methanol conversion. Unfortunately, small pore zeolites are rapidly deactivated during the conversion process. Rapid deactivation can be avoided and high selectivity to light olefins maintained by using a catalyst with a larger pore size but also with lower acidity. Such catalysts still have the downfall of producing undesirable aromatic by-products.
In order to avoid both rapid deactivation of the catalyst and the production of undesirable by-products, catalysts are needed which have both small pore size and intermediate acidity.